My infant, Jaden (girl) is losing weight. I know it is normal right after birth, but losing at 2-3 weeks!?

She was 7lbs 7oz at birth, and 7lbs 1 oz when we left the hospital. When we went in for the 2 week appointment, I couldn't get in until she was 2 1/2 weeks, and she was only 7lbs 4 oz. The doctor was concerned she wasn't up to birth weight, so I went back two days later after feeding her for 15 mins, alternating sides, and she was down to 7lbs 2 oz!! She has been having fewer diapers now (mostly poopy and green, not just wet ones). She also has thrush, so we started an anti-fungal for her, but just wondering if anyone has any suggestions? I was timing her feedings, and honestly have been feeding 16-19 times a day (it seems like all I do!), and she doesn't seem to be gaining! She is very alert, awake, and mostly happy (since she cries when she is hungry, but is sucking so much!). Thankfully my have finally toughened up! It was hard going for the first few weeks. Any suggestions? I don't want to suppliment with formula if I can help it, but that is the next step...